The present invention relates to an optical receiver module, and particularly to an optical receiver module with a high transmission rate of approximately 10 Gbits/s.
An optical receiver module in which a semiconductor photodiode is used as a receiver element is one of the key devices of transceivers for optical fiber transmission. With the recent spread of broadband networks, the optical receiver modules have increased in speed and the optical receiver modules with a bit rate of up to 10 Gbits/s are beginning to be widely used. The optical receiver modules suitable for the above-mentioned application are strongly demanded to be downsized, to be manufactured at low cost, and to be of low power consumption as well as to realize a good frequency bandwidth characteristic.
JP-A No. 2004-254125 discloses an optical receiver module in which a semiconductor photodiode and a preamplifier are mounted in a TO package to realize downsizing and low cost, and the impedance with respect to a high-speed signal at 10 GHz is decreased by reducing the inductance applied to a ground pad of the preamplifier, thereby securing a good frequency characteristic.
There are two cases as user demands depending on a design system of a transceiver in, particularly, the application to long distance transmission: one case is that linearity is required for receiver modules; and another case is that high receiver sensitivity is required for receiver modules.
In the case of making the power consumption of the module lower in the prior art, there arises a need to produce plural kinds of semiconductor photodiodes whose designs are changed in accordance with the kind of preamplifier to be mounted. As a result, there has been a problem that the usability of a semiconductor wafer is deteriorated, which makes it difficult to manufacture the semiconductor photodiodes at low cost.
Two power sources with positive and negative of −5.2V and +3.3V are conventionally available as power sources of a preamplifier in application to 300-pin MSA transceivers, and there is a sufficient margin in operating voltage. Therefore, it is possible to satisfy the both requirements with the use of one kind of preamplifier. To the contrary, for requirement of low power consumption in application to XFP transceivers that are becoming a mainstream, the driving voltage of the receiver modules is lowered to +3.3V of a single power source.
In the case of the single power source of +3.3V, it is difficult to design a preamplifier which satisfies the linearity and the high receiver sensitivity at the same time. For that reason, there arises a need to use different kinds of preamplifier ICs, i.e., a preamplifier IC with linearity and a preamplifier IC with high receiver sensitivity, whose designs are different from each other in receiver modules.
According to the study by inventors, the input impedance of a preamplifier with two power sources is about 30Ω (ohm). On the contrary, along with the requirement of the single power source of +3.3V, the preamplifiers are differentiated also in terms of input impedance. That is, the preamplifier with high linearity has a high input impedance of about 70Ω along with low gain. On the other hand, the preamplifier with high receiver sensitivity has a low input impedance of 20Ω to 30Ω along with first stage low noise.
In the case where the input impedance of the preamplifier is approximated by an input resistance Rin, a high-frequency equivalent circuit from a photodiode to an input portion of a preamplifier is as shown in FIG. 1. In FIG. 1, Cpd, Rpd, and L represent, a capacitance of the photodiode, a series resistance of the photodiode, and an inductance configured by a bonding wire, respectively. In order to obtain a desired good high frequency characteristic in the optical receiver module, the values of Rpd, Cpd, and L are selected to optimize the frequency dependency (i.e., angular frequency ω (omega) dependency) in which a photocurrent Iph (ω) generated at the photodiode generates a voltage Vin (ω) at both ends of the input resistance Rin.[Formula 1]Vin(ω)=Iph(ω)*Rin/(1+jωCpd*(Rin+Rpd+jωL))  (1)
On the basis of Formula 1, there will be examined a case in which the frequency characteristic of the optical receiver module is optimized by using, for example, the preamplifier with high receiver sensitivity having an input impedance of 30Ω and the preamplifier is changed for the preamplifier with high linearity having an input impedance of 70Ω. In this case, it is understood that Rpd and L are multiplied by 2.3 (70/30) in proportion to Rin, and Cpd is multiplied by 0.43 (30/70) in inverse proportion to Rin. L is easily changed with a bonding wire length. On the other hand, since a light receiving area and the capacitance Cpd are substantially in a proportional relation and the light receiving area and the series resistance Rpd are substantially in an inversely-proportional relation in the photodiode, it is necessary to reduce the light receiving diameter by 0.65 times (√/(30/70)) in order to satisfy both Rpd and Cpd.
As described above, in order that two kinds of preamplifiers that are different in the input impedance are separately used in the optical receiver module to obtain the same good frequency characteristic in each case, it is necessary to prepare two kinds of light receiving elements that are different in the light receiving diameter in a structure of a conventional optical receiver module. However, the light receiving element that is an important part, especially, an avalanche photodiode for long distance use is expansive. From the viewpoint of manufacturing photodiodes, there is a strong demand that the kind of photodiode is standardized irrespective of the kind of preamplifier so as to enhance the usability of a semiconductor wafer, to reduce chip unit cost, and to cut back production-inventory.
However, the invention disclosed in JP-A No. 2004-254125 fails to satisfy the demand, and there arises a problem that it becomes difficult to lower the cost of the semiconductor photodiode and furthermore the optical receiver module.